Since a fiber-reinforced composite material (hereinafter referred to as “FRP”) is lightweight and has high strength and high rigidity, it has been used in a wide range of fields including sports and leisure applications such as fishing rods and golf shafts and industrial applications such as automobiles and aircrafts. For manufacturing FRP, a method of using an intermediate material (prepreg) impregnated with a resin in a fiber reinforcing material layer made of long fibers of reinforcing fibers or the like is suitably used. A molded article made of FRP can be obtained by cutting the prepreg into a desired shape and then shaping, and curing this cut prepreg under heat and pressure.
In the field of aircraft, high mechanical characteristics such as heat resistance and impact resistance are required. Generally, a prepreg using an epoxy resin can provide a molded product having high mechanical characteristics, but requires a long molding time. Therefore, a carbon fiber-reinforced composite material having high mechanical characteristics such as heat resistance and impact resistance and being able to be molded in short time is desired.
In the press molding capable of short-time molding, high-temperature and high-pressure conditions of 100 to 150° C. and 1 to 15 MPa are usually used (Patent Literature 1). This high-temperature and high-pressure condition can shorten the curing time of a resin constituting a prepreg. Further, gas contained in the prepreg can be discharged by making the resin constituting the prepreg in a mold flow appropriately. However, when press molding is performed under the high-temperature and high-pressure conditions, the temperature of the resin constituting the prepreg rises and the viscosity of the resin remarkably decreases. As a result, depending on the structure of the mold, the outflow of the resin from a sheared edge portion occurs severely (hereinafter, the phenomenon that resin flows out of the prepreg by heating and pressurizing in the molding process is also referred to as “resin flow”). Therefore, the obtained FRP has a portion not impregnated with the resin composition (resin withering) and poor appearance such as fiber meandering, and a poor performance due to them.
In Patent Literature 2, there is described a method of suppressing resin flow by using a high-viscosity epoxy resin or adding a thermoplastic resin to an epoxy resin. However, when a high-viscosity epoxy resin is used, the resin viscosity at room temperature (25° C.) also increases. Therefore, the handling properties of a prepreg are remarkably low, such as difficulty of a laminating work.
In Patent Literatures 3 to 5, there are described prepregs for high-cycle press molding in which the handling properties of a prepreg at room temperature are improved and resin flow is suppressed without lowering Tg and a curing rate. In the resins used in the prepregs described in Patent Literatures 3 to 5, resin viscosity is increased by dissolving a thermoplastic resin in a liquid epoxy resin. However, since the resin viscosity at the time of preparing the prepreg also increases, there is a case where the impregnation properties of the resin into a reinforcing fiber base material layer are lowered, and thus voids may be formed in the FRP after molding.
Further, in the field of aircraft, mechanical characteristics such as high heat resistance and impact resistance are necessary, and thus various methods have been proposed for the purpose of improving impact resistance and interlayer toughness. Particularly, many techniques in which a material different from a matrix resin is interposed between layers to absorb fracture energy have been proposed (Patent Literature 6). However, the curing time of a resin generally takes 120 minutes or more, and it is difficult to perform short-time molding.
In conventional resin compositions, a resin composition in which the viscosity of an epoxy resin in an optimum molding temperature zone for short-time molding can be controlled and which can obtain a molded product having high mechanical characteristics such as heat resistance and impact resistance, and a prepreg using the resin composition have not been found. Therefore, it is required to develop a prepreg using an epoxy resin, which can be applied to short-time molding and can obtain a molded product having high mechanical characteristics such as heat resistance and impact resistance.